The invention relates to a transport container for nuclear fuel assemblies and in particular to a transport container for new fuel assemblies intended for refuelling a pressurized water nuclear reactor.
Nuclear reactors such as pressurized water nuclear reactors have a core made up of nuclear fuel assemblies which are of right prismatic shape and generally slim with a square cross section. The fuel assemblies generally have a cross section having the shape of a square, the side of which has a length close to 20 cm, the length of the fuel assembly, in its longitudinal direction, being about 4 meters. The fuel assemblies have a framework inside which nuclear fuel rods are arranged, essentially along the entire length of the fuel assembly. The framework itself is made up of spacer-grids which transversely hold the rods distributed over the length of the fuel assembly, of guide-tubes parallel to the rods engaged in the spacer-grids and of fuel assembly end nozzles.
Prior to reactor startup, the core needs to be fuelled with new fuel assemblies. Some core assemblies also need to be replaced after a certain period of time. New fuel assemblies need to be loaded into the core or substituted for used fuel assemblies which are removed from the core of the nuclear reactor. It is therefore necessary to have available new fuel assemblies which must be transported from the fuel fabrication plant to the nuclear power station in which the core of a nuclear reactor is being fuelled or refuelled.
The transport of new fuel assemblies, which is carried out by rail or by road, requires the use of transport containers which ensure effective protection of the fuel assemblies whose rods are not protected laterally between two successive spacer-grids. The transport containers must also be designed to avoid the destruction or even limited deterioration of the fuel assemblies, should the container be dropped, for example during a container transhipment manoeuvre during transport.
Fuel assembly transport containers are known from EP A, 506,512 and U.S. Pat. No. 5,481,117 which comprise an external envelope made from sheet metal made in the shape of two almost semi-cylindrical half-shells brought together and fixed one on top of the other along a rectangular frame arranged in an axial plane in the diametral direction of the container. The container is generally designed for the transport of two fuel assemblies and has a frame on which two fuel assemblies can be fixed, which rests on a cradle fixed via shock-absorber support elements inside the lower half-shell of the external envelope of the container. The frame for supporting and holding fuel assemblies is mounted such that it pivots on the cradle, by one of its ends, so that it can be moved between a position for loading fuel assemblies, in which position the support is substantially vertical, and a transport position in which the frame for supporting fuel assemblies rests on the cradle in a substantially horizontal position.
The frame for supporting fuel assemblies generally has a T-shaped transverse cross section which has a base for supporting fuel assemblies and a median wall in the longitudinal direction perpendicular to the base. The base supporting the fuel assemblies and the median wall define, on either side of the median wall, two housings into each one of which a fuel assembly can be placed. The fuel assemblies are held in the frame, via flanges articulated on the lateral edges of the base and on the upper edge of the median wall of the frame, so that the flanges can be moved between an open position in which the fuel assembly housing is accessible and a closed position in which the flanges hold the fuel assembly. The flanges are assembled to one another, in their closed position, by screw and nut assemblies and are arranged over the length of the frame so as to rest against the fuel assemblies placed in the housings of the frame, at each successive spacer-grid of the fuel assemblies.
The transport containers are designed so that the fuel assemblies placed side by side in the transport position cannot at any time form a critical mass leading to the initiation of neutron chain reactions. It is generally necessary to place neutron-absorbing elements between the fuel assemblies in the transport position inside the container, in order to avoid any risk of criticality.
Furthermore, it is also necessary to limit as much as possible the risks of the spreading of the fissile material contained in the fuel assemblies, should the fuel assembly undergo deterioration or destruction leading to the rupture of the protective cladding of the fuel rods, for example as a consequence of dropping the container.
In the case of transport containers of the known art, it is known that neutron-absorbing insulation means for fuel assemblies could be inadequate and that the containers do not have a structure enabling effective containment of the fissile material should the fuel assembly deteriorate inside the container. This is because the fuel assembly is neither protected nor contained inside the external envelope of the container, the frame only having means of holding fuel assemblies, in the form of flanges spaced over the longitudinal direction of the fuel assemblies, in the same way as the spacer-grids.
Furthermore, simulations of the dynamic behaviour of the containers and fuel assemblies, should the fuel assembly container be dropped in the axial direction or should it be dropped flat, have made it possible to show that highly effective energy absorbers need to be available to guarantee the mechanical integrity of the frame and of the fuel assemblies contained in the container envelope, should this container be dropped.
It therefore appears desirable to have available transport containers which ensure improved protection for fuel assemblies.
The object of the invention is therefore to provide a transport container for nuclear fuel assemblies of right prismatic shape, which has an external envelope and an internal structure defining at least one housing for receiving and holding a fuel assembly, having lateral faces arranged over a right prismatic surface and an end face at each longitudinal end of the housing, this transport container providing effective protection for the transported fuel assembly or assemblies and containing the fissile material contained in the fuel assemblies in order to prevent the fissile material spreading inside the external envelope of the container should the fuel assemblies deteriorate or be destroyed.
For this purpose, the internal structure of the container has a reception and holding unit for at least one fuel assembly having a support frame for at least one fuel assembly comprising at least two support walls for two lateral faces of a fuel assembly and two pivoting end walls for holding the longitudinal end parts of the fuel assembly as well as at least one door mounted such that it pivots on the frame between an open position to give access to the fuel assembly housing and a closed position in which the door, with the end walls and the bearing walls of the frame, ensures complete closure of a fuel assembly housing and protection and containment of the fuel assembly, independently of the external envelope.
The internal structure constitutes a case for receiving at least one fuel assembly, which case may be opened to give access to the fuel assembly housing.